CBM Versus NDS

We’re going to be stuck with both CBM and NDS for a long time. The latter is much more flexible, (e.g., allowing docking to an unmanned facility), but the former will stick around for its ability to transfer large objects.

Note that Dragon can’t serve as a lifeboat currently, because it has to have someone in the station, with power, to unberth from the CBM, even though it’s functionally capable of doing so with a rudimentary life support system. One of the key changes for commercial crew will be adoption of the NDS. One more reason that we should be accelerating that capability, because a Dragon lifeboat would allow the addition of another crew member, doubling or maybe even quadrupling the science that could be performed at the station.

I discuss this issue in the book:

To get back to the bizarre (at least that’s how it would appear to a Martian) behavior with respect to ISS, what is it worth? Of what value is it to have people aboard? We have spent about a hundred billion dollars on it over almost three decades. We are continuing to spend two or three billion a year on it, depending on how one keeps the books. For that, if the purpose is research, we are getting about one person-year of such (simply maintaining the facility takes a sufficient amount of available crew time that on average, only one person is doing actual research at any given time). That would imply that we think that a person-year of orbital research is worth two or three gigabucks.

What is the constraint on crew size? For now, not volume, though the life support system may be near its limits (the US segment can supposedly support four, and the Russian segment three) – I don’t know how many ultimately it could handle, but we know that there is currently not a larger crew because of NASA’s lifeboat requirement, and there has to be a Soyuz (which can return three) for each three people on the station. If what they were doing was really important, they’d do what they do at Scott-Amundsen, and live without. After all, as suggested earlier, just adding two researchers would immediately triple the productivity of the facility. In fact, because the ISS has recently been unable to average more than twenty-seven hours per week1, adding one person for a forty-hour week would increase it by two and a half times, and adding a second would increase it by a factor of four. If what we’re getting from the ISS in terms of research is really worth three billion a year, then quadrupling it would be, at least in theory, a huge value.

That’s not to say that they couldn’t be continuing to improve the safety, and develop a larger life boat eventually (the Dragon is probably very close to being able to serve as one now, since it doesn’t need a launch abort system for that role – only a new mating adaptor that allows it to dock to or depart from an unmanned or unpowered station), but their unwillingness to risk crew now is indicative of how unimportant whatever science being done on the station really is.

I should note that last week, the station did manage a record seventy-one hours, but I don’t think they’ll be able to keep that up with current crew size.

I wonder if CBM could be modified to remove the power requirements. When you say berthing, I assume that means that, unlike APAS, you can’t just ram two CBM modules into each other, but you need something like the ISS arm, which grabs one module and pushes it into the other. I wonder if you could eliminate/”fix” that requirement too.

I wrote the above before reading this post and the comments, so you partially answered my question, by answering Al’s. In theory, then, you could probably alter the design to allow release from either side, eliminating the “someone has to be on station” requirement. I would imagine you probably couldn’t “fix” the existing modules, though.

I prefer the idea of CBM over what little I’ve seen of NDS, because, thinking forward to when we have artificial gravity, I want to be able to WALK, not float, from one module to another, like Star Trek or Firefly.

It is more than just power. The mechanism is controlled via four Control Panel Assemblies (CPAs) on the ISS side. Due to the size of the hatch opening, the CPAs physically obstruct the hatch. They must be uninstalled and removed by the crew after berthing but prior to hatch opening, and must be reinstalled by the crew (from the inside) after hatch closure but prior to unberthing. This takes quite a bit of time.

CBM was designed for permanent and semi-permanent connections between modules with a large hatch. It simply wasn’t designed for undocking, and especially not for emergency undocking, which requires a mechanism that can be operated quickly from the spacecraft side without ISS power. People should stop trying to turn it into something that it’s not, and accept that manned spacecraft will use docking mechanisms.

To expand further, originally station resupply was to be handled by the shuttle, using the Multi-Purpose Logistics Modules (MPLMs). The MPLMs used CBM so that they could transfer large-diameter cargo. There was no requirement to remove an MPLM quickly – if a shuttle had to do an emergency undocking, it would simply leave the MPLM behind.

The idea of using CBM on free-flying cargo spacecraft, such as HTV, Dragon, and Cygnus, came along later, and picked up momentum once the decision was made to retire the shuttle. A historical accident, in other words. CBM isn’t a docking mechanism and it’s a mistake to think of it as one.

…because it has to have someone in the station, with power, to unberth from the CBM,

That is a little silly as they already have two life boats.
A Soyuz and a Dragon could return up to ten people with one who will use the Soyuz to assist with the Dragon’s hatch.
What we really need is to get past the politics and get a Dragon with seats up there as the second lifeboat then increase the crew size.

This is 2013, isn’t it? I had to check. We’ve been mating spacecraft since the 60’s or 70’s and haven’t resolved this yet? You start with specified requirements. Then you hand it to engineers. Then you pick a design and everybody agrees to use it.

* No genders. All are identical and will mate with each other.
* They work with power or without.
* They are self aligning. Get within a certain distance and they take care of making the final seal. No ‘impact’ required.
* They are of sufficient size to transfer a standard sized cargo container (and obviously suited crew.)

Getting designs to fit the specification should be the easy part… Then you lock them in a room like a bunch of cardinals until they choose one design.

“…For that, if the purpose is research, we are getting about one person-year of such (simply maintaining the facility takes a sufficient amount of available crew time that on average, only one person is doing actual research at any given time)…”

Makes me wonder why we insist on the non-specialization model for the crew. In other words, we might get much more productivity if we have one or two techs whose job it is to do “maintenance” and let the other folks concentrate on the science. If you have permanent “crew” I’d think the elitists would lower the life boat requirement to just cover the special people. Let the “hired help” fend for themslves.

It’s not really a skill set issue — it just takes that many people to do the maintenance, but that is a good point, and one I should make in the book. Traditionally, if there are life boats, the passengers get first dibs and the crew, and particularly the captain, go down with the ship.

When I worked maintenance requirements for ISS, someone noted to me that the Russians tended to select Cosmonauts that were good technicians and then trained them to perform science. Americans tended to select Astronauts and then train them to perform maintenance. As I was going through Kranz’s book, it seemed we used to select technicians (test pilots) and train them to perform science up until Apollo 17.

Let’s also not forget several times a year, for from several weeks to over a month, the crew size drops down to 3. So in effect, we have about 5 to 5-1/2 crewmembers per year. Just having the crews overlap would be an improvement.

I may be wrong about this, but I believe a visiting vehicle can undock from ISS on its own anyway, by opening the active structural latches on its NDS, then firing its NDS passive-latch release pyros to free them from any closed ISS-side active structural latches. In theory, no harm, no foul for reuse of the commercial vehicle’s NDS because all of the commercial outfits plan to discard NDS prior to entry. Now that’s a real waste of $$$!